The purpose of this study is to evaluate causal association between neuropsychological executive dysfunction and HIV infection among young injection drug users (IDUs) through two linked studies. First, a case- control study will be conducted to compare the baseline neuropsychological performance of 11 HIV-positive and 400 HIV-negative IDUs. The primary aim of the case-control study is to estimate the strength of association between executive dysfunction and being HIV infected using logistic regression modeling to adjust for suspected confounders, including CD4 counts. Second, a longitudinal study would be conducted in which the cohort of 400 seronegative IDUs completing the baseline neuropsychological battery will be re-assessed on three subsequent occasions, roughly six months apart, and the data analyzed primarily by GLM/GEE models. The primary aim of the longitudinal study is to estimate the magnitude of the suspected causal relationship between executive dysfunction and HIV-risk behaviors while adjusting for time-invariant (e.g. sex, ethnicity) and time-varying (e.g. degree of drug abuse) covariates. In the longitudinal study, we also seek to evaluate: (1) the degree to which specific executive dysfunctions predispose young IDUs to high-risk injection practices or sex behaviors, and (2) whether observed relationship between executive dysfunction and HIV-risk behaviors can be understood independent of levels of drug -taking frequency, or whether the observed data are more consistent with complex patterns of interdependency between executive dysfunction, drug-taking frequency, and HIV-risk-behaviors. The proposed study benefits substantially from linkages to NIDA- and CDC-funded studies of young IDUs. To date, our research team knows of no published report examining neuropsychological deficits that signal HIV-risk behaviors. If sucessful, this project will shed new light on significant and potentially malleable HIV-risk factors in young IDU initiates. This will be important evidence because injection drug abuse continues to account for a large proportion of HIV seroconversions particularly among young women and minorities. As such, this RO1 research project serves as an important initial step in a line of innovative investigations about suspected causal associations between neuropsychological deficits and HIV-risk behaviors in IDUs. Ultimately, this line of investigation should lead to changes in public and clinical practices designed to prevent HIV infection.